1. Field of the Invention
The invention relates generally to the field of digital data processing systems and more specifically to networks for use in connection with distributed systems.
2. Description of the Prior Art
A digital data processing system, or computer, typically includes a processor, associated memory and input/output units enabling a user to load programs and data into the computer and obtain processed data therefrom. In the past, computers were expensive, and so to be cost effective had to support a number of users. More recently, however, the cost of computers, particularly the processors and memories, has decreased substantially, and so it is relatively cost effective to provide a computer to one or at most only a few users.
A benefit of providing only a single computer for a large number of users was that the users could easily share information. Thus, for example, if all persons working in a bookkeeping or accounting department use a single common computer, they may maintain common accounting and bookkeeping databases up to date, and when necessary accounting reports may be generated from those databases. However, if they use separate computers, the data is stored in separate databases, on each computer, and so generating accounting reports would be more difficult.
As a result, networks were developed to provide a distributed computer system, that is, a system which permits diverse computers to communicate and transfer data among them. In addition, the networks allow the sharing of expensive input/output devices, such as printers and mass storage devices, and input/output devices which may be rarely used, such as links to the public telecommunications network. In a network, each computer is a node which communicates with other nodes over one or several wires. In addition, nodes may be provided which store and manage databases or other data files on mass storage devices, or which manage printers or links to the public telecommunications network.
A problem arises with networks, however, since each computer manufacturer essentially has a different protocol for allowing nodes to communicate over the networks. For example, computers from International Business Machines, Inc., communicate using a System Network Architecture ("SNA") protocol standard, computers from Digital Equipment Corporation communicate using a Digital Network Architecture ("DNA") protocol standard, and Univac computers communicate using a DCA protocol standard. In an effort to establish protocol standards to allow computers and other nodes from diverse manufacturers to communicate, the International Standards Organization (ISO) has established an Open Systems Integration (OSI) protocol standard.
One similarity among all of the protocols is that each is divided into a plurality of layers, with each layer providing services to the communication. For example, the OSI protocol includes seven layers. Low layers control the generation and reception of electrical signals over the communications medium to allow the communication of individual bits, the sequencing of the bits into blocks, and the determination of the node which is to respond to the signals. Intermediate layers control the sequencing of blocks of received bits into messages of data in case the blocks are received out of order, and the division of data messages into blocks for transmission over the network if the network cannot transmit an entire message in a block. In addition, another intermediate layer controls the transfer of data to and from the proper programs which may be running on the node. Upper layers control the transfer of data to and from the programs running on the node and the addition of error detection and correction information and its decoding to ensure proper receipt of the data. While the protocols are all divided into layers, the services provided by each of the layers are diverse, as is the mechanism for communicating among the layers.